Particularly in the last decade, liquid crystals have been introduced into various industrial areas in which electro-optical and display-device properties are required (for example in watch, calculator and typewriter displays). These display devices are based on dielectric alignment effects in the nematic, cholesteric and smectic phases of the liquid-crystalline compounds, where--caused by the dielectric anisotropy--the molecular long axis of the compounds adopts a preferential alignment in an applied electric field. The usual response times in these display devices are too long for many other potential areas of application of liquid crystals. This disadvantage is particularly noticeable if a large number of pixels must be addressed. Production costs of equipment containing relatively large screen areas are then generally too high.
In addition to nematic and cholesteric liquid crystals, optically active smectic liquid crystals have also been increasing in importance for the last few years.
Clark and Lagerwall have shown that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in electro-optical switching or display elements which have response times faster by a factor of 1,000 compared with conventional TN ("twisted nematic") cells (cf., for example, Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, October Meeting 1985, San Diego, Calif., USA). On the basis of this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are in principle highly suitable for the abovementioned areas of application, for example via matrix addressing. Ferroelectric liquid crystals are also particularly suitable in the area of spatial light modulators (cf., for example, U. Elton in "Spatial Light Modulators and Applications", SPIE, Vol. 1150, pp. 46 ff) due to their high contrast and fast response speed. However, the response speed of ferroelectric liquid-crystal mixtures is generally still not sufficient to drive, for example, high-resolution, fast display elements. It is therefore desirable to find components which increase the response speed of liquid-crystalline mixtures.
The molecular structure of the liquid crystals which can be employed for the abovementioned areas of application is essentially based on para- or 1,4-substituted aromatic and saturated six-membered rings, such as 1,4-phenylene, pyridine-2,5-diyl, pyrimidine-2,5-diyl and trans-1,4-cyclohexylene, which, through linking to one another, either directly or via suitable intermediate pieces, and by linking to suitable terminal groups give the elongate rod-shaped molecules known from numerous examples (see, for example, in: D. Demus, H. Zaschke, "Flussige Kristalle in Tabellen I und II" [Liquid Crystals in Tables I and II], VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig 1974 and 1984).
By contrast, liquid-crystalline compounds which contain, as substructure, an exclusively meta- or 1,3-substituted six-membered aromatic ring have hitherto not been described, since it was expected that the use of such a unit would disturb the rod-shaped molecule geometry and would thus not give any liquid-crystalline properties.